Technical Field
Embodiments of the present invention generally relate to an electrophotographic image forming apparatus, such as a copier, a printer, a facsimile machine, and a multifunction peripheral (MFP) having at least two of copying, printing, facsimile transmission, plotting, and scanning capabilities.
Description of the Related Art
Electrophotographic image forming apparatuses typically include a charger to uniformly charge a surface of a latent image bearer, an exposure device to expose the latent image bearer with light according to image data, thereby forming an electrostatic latent image thereon, and a developing device to develop the electrostatic latent image into a toner image (visible image).
In such image forming apparatuses, image density tends to fluctuate depending on changes in ambient conditions (temperature and humidity) under which the apparatus is used or changes with elapse of time. There are image forming apparatuses that execute so-called process control to attain a stable image density. In the process control, a gradation pattern for image density adjustment is formed on the latent image bearer, and the image density of the gradation pattern on either the latent image bearer or a transfer medium is detected, and image forming conditions are changed according to detection results of the gradation pattern.
Specifically, in the process control, an optical detector (hereinafter “optical sensor”) detects the image density of the gradation pattern, and the amount of toner adhering to the gradation pattern is obtained from the detection value output by the optical sensor. Additionally, a developing potential is obtained from an exposed area potential and a developing bias. The exposed area potential means the potential of the electrostatic latent image, which is formed on the latent image bearer to form the gradation pattern. Then, a formula representing the amount of toner adhering (Y axis) relative to the developing potential (X axis) is established as y=ax+b, wherein inclination a represents developing γ, and X axis intercept represents development start voltage Vk (development start voltage).
After the process control is executed, based on the above-mentioned formula, image forming conditions, which include exposure (exposure light amount), a charging bias, the developing bias, and the like, are changed. With this control, the image density is stabilized even when there are changes in ambient conditions (temperature and humidity) or changes with elapse of time.
The exposed area potential in formation of gradation pattern is extracted from the detection results generated by a potential sensor to detect the surface potential of the latent image bearer. For example, the exposed area potential is extracted as follows. Based on a layout distance from an exposure position, where the latent image is written, and a potential detection position of the potential sensor and the surface movement speed (i.e., rotation speed) of the latent image bearer, the exposed area potential is extracted from the detection results of the potential sensor synchronized with the timing at which the latent image of the gradation pattern reaches the potential detection position.
In another example, the exposed area potential is extracted using a potential threshold predetermined in a range between the potential of an unexposed area after uniform charging (hereinafter “unexposed area potential”) and the exposed area potential. In this method, the surface potential of the latent image bearer is detected consecutively at regular cycles. The exposed area potential is calculated using the detection results after elapse of a predetermined time after the detection result falls below the potential threshold in the polarity of charged potential.